US Supply Lines Threatened in Iraq?

Several months ago, I speculated that Iraq was entering a civil war. I noted then the US military's movement of large gunships to Iraq and speculated that the reason was to provide cover for any possible mass retreat. ...

Now that many are saying there's a de facto civil war underway, commentators are noting the threat to US Army personnel should the civil war break out into house-to-house fighting. Most threatened are the supply lines that feed and provide weapons to US troops in Iraq.

At Obsidian Wings, andrew provides some review and related comments of what might turn into a dire situation for US troops in Iraq:

As Billmon and Lang note, the U.S. Army in Iraq currently depends on ground transport to provide it with the supplies it needs to keep its various Forward Operating Bases (FOBs) running. To date, the Iraqis have generally left those convoys alone, but they represent by far the most vulnerable aspect of the American presence in Iraq. Iraqi attempts to storm U.S. FOBs would have poor chances for success, but blocking the roads could accomplish much the same thing without the need to risk thousands of Iraqi lives. With the roads shut down, the U.S. would quickly run short of supplies and would have to shift its focus to force protection.

As I noted above, such a shift in the fight would mark the end of any U.S. attempts to support the Iraqi government. Facing the threat of losing most of the ~135,000 U.S. troops in Iraq, U.S. leaders would have to worry about either consolidating U.S. forces into a few, easily defended FOBs, or extracting them completely from Iraq. Since, in either case, we would no longer be able to conduct counterinsurgency (COIN) operations, that would mark an end to the U.S.'s ability to influence events in Iraq.